Add config option to disable programming ligatures

[zokrezyl]

Summary

There is currently no runtime way to disable programming ligatures. They are gated purely by the compile-time YETTY_ENABLE_LIB_HARFBUZZ feature flag — when harfbuzz is compiled in, the bundled FiraCode ligature face is always loaded and ligature runs are always shaped. Turning ligatures off requires rebuilding without harfbuzz.

Current behaviour

• All ligature handling is wrapped in #ifdef YETTY_ENABLE_LIB_HARFBUZZ (src/yetty/yvterm/vterm.c).
• When harfbuzz is enabled, vterm_load_ligature_face(vterm, config) is called unconditionally during font setup (src/yetty/yvterm/vterm.c:1259-1263). No config key is consulted.
• A search of src/yetty/yconfig and include/yetty/yconfig finds no ligature-related key.
• The only "off" path is a best-effort fallback: if the FiraCode face fails to load, it logs ligatures disabled and renders operator characters individually — a failure path, not a user setting.

Proposal

Add a runtime config key (e.g. terminal/text-layer/font/ligatures, default true to preserve current behaviour) and guard the vterm_load_ligature_face() call on it. When disabled, skip loading the ligature face so ligature_face_index stays 0 and each character renders individually — no rebuild required.

Sketch

• Add YETTY_YCONFIG_KEY_... for the new key in yconfig.
• In src/yetty/yvterm/vterm.c:1262, wrap the call:

  if (config->ops->get_bool(config, YETTY_YCONFIG_KEY_TERMINAL_FONT_LIGATURES, true)) {
      vterm_load_ligature_face(vterm, config);
  }

• Document the key in the config docs.